I just want to download the full 1903 patch for Windows server 2019 and can't find it anywhere. It keep redirecting to Window 10. I am sure they are based of the same OS, but there should be a way to download it manually or force a server to use it.

If you're running Semi-annual Channel releases (1803/1903) then you can in-place upgrade between releases. But otherwise you should plan to redeploy your Windows Server K8s hosts when you require OS upgrades.